1. Introduction
This invention relates to etchants comprising a synergistic combination of hydrogen peroxide and molybdenum as oxidants in acidic solution.
2. Description of the Prior Art
The activation of relatively dilute mineral acids with an oxygen release compound is known in the art and described, for example, in Plating, "Surface Treatment of Metals with Peroxygen Compounds", Vol. 42, pg 561 (1955). In the Plating publication, it is taught that mineral acids such as sulfuric acid, nitric acid, acetic acid and phosphoric acid may be activated with oxygen release compounds such as hydrogen peroxide or other peroxy, persulfate or perborate compounds. The oxygen release compound enhances the etch rate thereby permitting a decrease in the concentration of acid.
Activation of dilute sulfuric acid with hydrogen peroxide is most attractive due to low cost and ease of electrolytic copper recovery from the spent etchant prior to disposal. However, the utility of such hydrogen peroxide etching solutions is reduced somewhat due to catalyzed decomposition of hydrogen peroxide caused by etched metal ions or other transition metal ions in solution and a slow etch rate, typically 1.4 mils of copper from 1 ounce copper clad laminate in 10-25 minutes at 120.degree. F. To enchance etch rate, more concentrated solutions of hydrogen peroxide have been used, but concentrated peroxide solutions are hazardous to health and safety. In addition, such etchants have not been successfully used with tin dissimilar metal etch resists because they attack tin, especially immersion tin.
To limit metal ion catalyzed decomposition of hydrogen peroxide, a number of stabilizers are used in the prior art. A variety of such stabilizers is disclosed in U.S. Pat. Nos. 3,293,093; 3,341,384; 3,407,141; and 3,668,131, all incorporated herein by reference. A preferred class of stabilizers is disclosed in U.S. Pat. Nos. 3,801,512 incorporated herein by reference. The stabilizers disclosed in said patent are the arylsulfonic acids.
Though stabilized acid solutions activated with hydrogen peroxide are improved over their unstabilized counterparts, nonetheless they still possess disadvantages which limit their use, particularly for the manufacture of printed circuit boards.